Perpendicular magnetic recording is a recording technique in which magnetic data bits on a magnetic recording disk are defined such that their magnetic moments are perpendicular to the plane of the magnetic recording disk, as opposed to in the plane of the disk as occurs with longitudinal magnetic recording. The switch to perpendicular recording from longitudinal recording is seen as one of the advances that will allow the continued increase in data densities on magnetic recording disks in the coming years.
FIG. 1 shows a cross-sectional view of a perpendicular recording system 100 according to the prior art. The perpendicular recording system 100 includes a magnetic recording disk 110 and a perpendicular recording head 120. The magnetic recording disk 110 includes two layers, a recording layer 130 and a soft magnetic layer 140 disposed beneath the recording layer 130. The perpendicular recording head 120 includes a main pole 150 and a secondary pole 160 magnetically coupled together. Both the main pole 150 and the secondary pole 160 are exposed at an air bearing surface (ABS) that faces the magnetic recording disk 110. The perpendicular recording head 120 also includes coil windings 170 disposed between the main pole 150 and the secondary pole 160.
When an electric current is passed through the coil windings 170, a magnetic field is induced in the main pole 150 and the secondary pole 160. Lines of magnetic flux 180 from the magnetic field extend between the exposed surfaces of the main pole 150 and the secondary pole 160 to complete the magnetic loop around the coil windings 170. The flux lines 180 extend through the recording layer 130 and into the soft magnetic layer 140 where they curve around to return back through the recording layer 130. The main pole 150 and the secondary pole 160 are configured such that the flux lines 180 are densely constrained at the exposed surface of the main pole 150 so that the density of the flux lines 180 are sufficient to define a magnetic bit 190 in the recording layer 130 where the flux lines 180 pass through the recording layer 130 beneath the main pole 150.
FIG. 2 shows a top view of a portion of the magnetic recording disk 110 after a series of magnetic bits 190 have been written. One problem with perpendicular recording is a tendency for the transitions 200 between successive bits 190 to be curved. This curvature occurs because of the shape of the magnetic field beneath the main pole 150. Although the main pole 150 defines a rectangle where exposed at the ABS, the magnetic field beneath the main pole 150 rapidly looses the rectangular shape as the flux lines 180 repel one another. Accordingly, the magnetic field takes on a shape of a rectangle with rounded sides where the magnetic field intersects the magnetic recording disk 110. The rounding of a trailing side of the magnetic field causes the transitions 200 to be correspondingly rounded.
It will be appreciated that, from the perspective of a magnetic reading element reading the magnetic bits 190, the curvature of the transitions 200 makes the time necessary to read each transition 200 longer than would be necessary to read a hypothetical straight transition. And, as the size of the individual magnetic bits 190 decreases, with increasing linear data density, the transitions 200 become a substantial portion of the length of each magnetic bit 190. If the size of the magnetic bits 190 is reduced far enough, the transitions 200 begin to overlap and the reading element can no longer distinguish one transition 200 from the next. Thus, curved transitions 200 limit linear data densities.
Hence, there is a need for a perpendicular recording head that is capable of writing magnetic bits with straighter transitions in order to achieve higher linear densities.